Conventionally, a vibration sensor for sensing earthquake vibration, combines two vibration systems having natural vibration frequencies in two frequency bands. The two system vibration sensor can detect frequencies higher than that of earthquake vibration to distinguish earthquake vibration from shock vibration caused by a collision of an object with equipment incorporating a vibration sensor.
FIG. 10 shows an example thereof. A vibration sensor 20 has a cone-shaped vibrating surface 23. Vibrating surface 23 has a recess 22 whose top end part is small in diameter, and is formed at the central part of the inner bottom surface of a case 21. A comparatively large steel ball 24 (i.e., vibrating member) reciprocates by rolling on this vibrating surface 23. A sliding piece guide 25 moves upwards in response to the rolling of the steel ball 24. Sliding piece guides 25a and 25b support the sliding piece 25 in a vertically slidable manner. A movable contact 26 is pushed up by the sliding piece 25, and a fixed contact 27 contacts the movable contact 26 when it is pushed up.
In the above-mentioned vibration sensor 20, when the steel ball 24, bound in the central recess 22 of the cone-shaped vibrating surface 23, is subject to vibration above a certain value in Gal, it moves out of the recess 22 and reciprocates on the vibrating surface 23. At this time, the movable contact 26 is moved upward by the rolling motion of the steel ball 24, and contacts the fixed contact 27. As a result, an ON/OFF signal is generated from leads 28.
The vibration system dominating the movement of the steel ball 24 of this conventional vibration sensor 20, as shown in FIG. 11, has a vibration system A and B. The vibration system A has a comparatively low natural frequency (for example, about 4 Hz) which is determined from the curvature of a spherical surface replaced by the vibrating surface 23 and the size of the steel ball 24. The vibration system B has a comparatively high natural frequency (for example, about 12 Hz) which is determined from the diameter of the top end part of the central recess 22 of the vibrating surface 23 and the size of the steel ball 24. Accordingly, when operating in the earthquake wave band (1-4 Hz), the steel ball 24 moves out of the recess 22 at the level of the vibration system B, and thereafter moves to system A operation because of the lower operation level of the vibration system A. The steel ball reciprocates on the vibrating surface 23. Also, when the steel ball 24 operates in the shock wave band (for example, 5 Hz or more) i.e., other than earthquake vibration, it moves out of the recess 22 at the level of the vibration system B and approaches the vibration system A. However, unless a vibrating force above the operation level of the vibration system A is applied, no reciprocating motion on the vibrating surface 23 occurs. This is because the operation level of the vibration system A is higher than the applied vibrating force.
However, the actual vibration at a shock or the like sometimes has a frequency close to the earthquake wave band, and even a frequency above 5 Hz sometimes causes a vibrating force exceeding the operation level of the vibration system A. Therefore, as shown in FIG. 12, a signal processing means 29 is connected to the leads 28 of the vibration sensor 20 to process an ON/OFF signal of the vibration sensor. When ON-time pulses above a predetermined time width (for example, 30 ms) are generated a predetermined number of times (for example, five times) within a predetermined time (for example 3 sec) and an OFF-time is above a predetermined time length (for example, 40 ms) the vibration is judged to be an earthquake vibration. The predetermined time width of each ON-time and OFF-time are set to a value which will not produce detection of shock wave or the like having a frequency component higher than an earthquake wave. However, since the lengths of ON-time and OFF-time disperse, each are set to have some margin. For example, in the case where the predetermined time widths of the ON-time and OFF-time are set respectively to 50 ms and 60 ms, the following holds: ##EQU1## and therefore, principally any vibration having a frequency above 5 Hz is not detected. But in actuality the lengths of ON-time and OFF-time disperse; and therefore, there is a fear that the main earthquake vibration might not be detected. Accordingly, the above-mentioned predetermined time widths of ON-time and OFF-time are set to 30 ms and 40 ms, respectively, to incorporate some margin of forgiveness. Thus from the following: ##EQU2## any vibration having a frequency above 7 Hz is not detected. Also, the number of ON-pulses during a predetermined time and the predetermined time, during which these pulses are counted, are set to values at which shock waves or the like having a frequency close to the earthquake wave band are not detected.
The above-mentioned conventional vibration sensor as has an unstable vibration because two vibration systems exist on the same vibrating surface. When the steel ball 24 is vibrating in the vibration system A, it passes through the central recess 22 of the vibrating surface 23. At this time it returns once to vibration system B operation, and this causes an interference between the two vibration systems. When the steel ball 24 is vibrating in the vibration system A, the small diameter of the top end part of the recess 22 causes the center of gravity of the steel ball 24 to act out of the range of the recess 22. Therefore when the steel ball passes through the recess 25 the direction of motion of the steel ball 24 is changed or a rotary motion is induced.
An earthquake detecting system also must discriminate between an earthquake wave and the vibration caused by a collision of an object with the equipment incorporating the vibration sensor. This characteristic is in addition to the function of reliably detecting the earthquake wave. Where the equipment incorporating the vibration sensor is firmly fixed, the vibration caused by a shock is small. The frequency of this vibration is high in comparison with an earthquake wave, and discrimination therebetween is easily made. On the other hand, when the equipment incorporating the vibration system is loosely fixed by piping or the like, the inadvertent vibration is large. The frequency of this vibration becomes close to that of the earthquake wave, and therefore discrimination therebetween becomes difficult. To detect the earthquake wave reliably and discriminate the earthquake wave from the shock wave, it is required that the frequency characteristic of the vibration sensor be flat in the earthquake wave band (1.4-3.3 Hz) and rise sharply before the shock wave band (for example, 5 Hz or more). It is difficult to realize such a characteristic using a single vibration system. The vibration system of the conventional vibration sensor, as described, attempts to accomplish discrimination by combining two vibration systems. Now, description is made of the characteristics of the vibration system of this conventional vibration sensor using FIG. 11. The natural frequency of the vibration system A is, for example, about 4 Hz, and the natural frequency of the vibration system B is, for example, about 12 Hz. When operating in the earthquake wave band, the vibration member 24 moves out of the recess 22 at the operating level of the vibration system B, having a gentle slope in this band, and thereafter it moves to the operating level of vibration system A because of the lower operating level of the vibration system A. The steel ball 24 reciprocates while turning on the contact 26. When the vibrating frequency of the steel ball 24 becomes high and the operating level of the vibration system A becomes higher than that of the system B, the steel ball 24 does not move from the vibration system B to the vibration system A by the same vibrating force. When the steel ball 24 does not vibrate in the vibration system A, the contact 26 is not turned on, and therefore the operating point moves to the vibration system A and rises sharply.
This linked vibration system has two vibration systems on the same vibrating surface, and therefore has an unstable vibration. The vibration member 24 has a degree of freedom of 360.degree. with respect to the vibrating direction, and therefore a slight disturbance to the vibrating direction is likely to make the vibration unstable. As shown in the conventional example, the recess 22 in the center of the vibrating surface 23, causes vibration member 24 to either change direction or begin a rotary motion as vibration member 24 pass over recess 22.
As a means for discriminating an earthquake wave from shock wave on the signal processing means 29 side, a method is adopted which sets a lower limit value of pulse width of the ON/OFF signal from the vibration sensor 20. An earthquake wave is not detected when pulses are below this lower limit value. Actually, however, the pulse width of the ON/OFF signal from the vibration sensor disperses, and the above-mentioned lower limit value is required to be set with a considerable margin. Therefore this method has no practical discriminating effect on shock waves close to the earthquake wave band.
In the actual shock test shown in FIG. 13, there is worse shock resistance before or after the disappearance of signal pulses occurring after gradual attenuation of the shock wave. This is caused where the movement of the vibration member 24 becomes weak and unstable, signal pulses are missed, the frequency is lowered equivalently, and the pulses are not filtered by the above-mentioned frequency filter.